An air turbine starter is a device used to start a turbine engine, such as a gas turbine jet engine commonly found on aircraft. The air turbine starter is connected to the jet engine and is used to start the jet engine in generally the same way as a starter for an automobile is used to start the automobile's engine. The developer of the present inventions, Honeywell International, Inc., has for years successfully designed, developed, manufactured and repaired air turbine starters.
FIG. 1 shows a partial cut-away diagram of a conventional air turbine starter 100, which includes an air inlet assembly 103 that is joined to a main housing 105. Maintained within the main housing 100 are airways and other components such as a turbine assembly 107, an air outlet 109, and a gearbox 111 which is coupled to an output shaft (not shown). The turbine assembly 107 has a turbine wheel 113 with circumferentially mounted blades 115, a rotatable drive shaft 117 and a gear 119. The air inlet assembly 103 is made up of two primary components, a stator 121 and an outer shell 123. In many instances the stator 121 and outer shell 123 provide mating threads 125. In some cases a locking pin 127 may additionally be used to assist in keeping the stator 121 and outer shell 123 together. Additional turbine starter features are disclosed in Honeywell's U.S. Pat. No. 6,318,958 (Giesler et al.) and U.S. Pat. No. 4,914,906 (Burch) which are incorporated by reference herein.
In order to start a jet engine the air turbine starter 100 is first activated. Generally speaking, such activation is accomplished by connecting an air pressure duct to an air inlet 129 provided by the stator 121 portion of the inlet assembly 103. Compressed air is directed by contoured passage 131 through stator fins 133, across the turbine blades 115 and is vented from air outlets 109. In operation, the energy of the moving air is converted by the blades 115 into rotary motion, causing the turbine assembly 107 to rotate.
Generally, the turbine starter 100 is joined to the jet turbine engine such that it travels with the jet. As a result, the weight of the turbine starter 100 is generally a calculated component of the overall weight of the aircraft and as such, reduces the total amount of cargo weight that the jet may transport. In the commercial aircraft industry, each additional pound of weight may cost the aircraft manufacturer a financial penalty. Likewise each additional savings of a pound may be credited to the manufacturer as a financial savings.
As noted above, the inlet assembly 103 is comprised of two components, namely the stator 121 and outer shell 123. The function of the stator fins 133 is to direct the supplied compressed air across the turbine blades. The narrowing passageways between the stator fins 133 act as nozzles to increase the velocity of the air as it strikes the rotating turbine blades 115. Given the velocity and pressure of the compressed air, it is generally desirable to align the direction of the air flow to the turbine blades 115 so as to reduce stress and wear upon the turbine assembly. The outer shell 123 generally aligns the stator fins 133 to the turbine blades 115 and provides the outer portion of the contoured passage 135 leading to the air outlets 109.
The manufacture of the air inlet assembly 103 is typically an involved tooling process given the nature of the air inlet 129, the contoured passage 131, and configuration of the stator fins 133. As the name suggests, the stator 121 and the stator fins 133 do not rotate. Typically the outer shell 123 may be fabricated as a single piece from a titanium alloy, desired for it's strength and relative light weight as well as other characteristics.
Manufacture of the stator 121 as a single item from a titanium alloy has heretofore not been achievable. The contours, airfoil shapes and limited spaces have frustrated attempts to produce simply the stator 121, let alone the outer shell 123 and stator 121 as a single contiguous item. As a result, the stator 121 is generally manufactured from a heavier, but easier to tool alloy such as an inconel alloy. Several machining steps may be needed to join the stator 121 to the outer shell 123, each step potentially resulting in additional training, equipment, cost, and time, as well as potentially different geographic locations of each step of fabrication—a factor adding yet further cost for time and shipping. In addition, the outer shell 123 may be flared out or fabricated with additional sidewall thickness in the area accommodating the mating threads 121. As such, the inlet assembly 103 weight as thickened may be greater than what could be achieved with a unitary inlet assembly. Further, as the outer shell 123 and stator 121 are fabricated from different metal alloys, the different relative hardness and thermal expansion and contraction properties may frustrate the threaded union and accelerate wear between the components.
Wear of the stator fins 133 and turbine blades 115 is understood to be a natural result of starter operation. In certain instances, internal vibration and or dynamic responses of the turbine blades may result in fracturing of the turbine blades 115, also known as mouse bites. The occurrence of occasional mouse bites to the turbine blades 115 may decrease operational performance, cause internal damage, and/or accelerate the need for maintenance. The common practice of setting the joined stator 121 and outer shell 123 with a locking pin 127 has been found to occasionally fail. Operational vibration of the aircraft, thermal expansion and contraction, and or perhaps even installation error may introduce the end 137 of the locking pin 127 into the contoured passage 131, an event that may or may not affect the performance of the starter. Should the locking pin 127 come loose during operation and entirely enter the passage 131, passage of the pin 127 through the stator fins 133 and or the turbine blades 115 may cause significant damage to these components and affect the overall function and performance of the turbine starter and may necessitate a more extensive rebuild of the turbine starter 100.
However, it should be appreciated that despite the drawback of mouse bites and the potential failure of the locking pin 127, air turbine starters are generally operationally safe and reliable. Inspections of the air inlet 129 and stator 121 are generally part of the routine maintenance schedules set for the turbine starter 100.
Hence, there is a need in for an improved air turbine starter having an inlet and stator with improved characteristics to overcome one or more of the drawbacks identified above. The present invention satisfies one or more of these needs.